This invention relates to a rotary drum type apparatus for applying a coating to tablets.
In apparatus for applying a coating to tablets, the tablets are placed in a rotary drum and agitated while a solution is sprayed onto the tablets to apply a coat of the solution to each tablet. At the same time, the solvent is allowed to dry to leave the coating of the solute on each tablet.
An apparatus for applying a coating to tablets of the prior art is shown schematically in FIGS. 1 and 2 which comprises a flat rotary drum 1 slightly bulging at the centers of the front and rear ends, a support frame 2, an air inlet tube 3 and an air outlet tube 4. The drum 1 which is the main body of the apparatus for applying a coating to tablets includes an outer periphery 8 formed of a porous air-permeable plate, and a door 5 attached to the front end for inserting the tablets into the drum 1 and withdrawing same therefrom.
A rotary shaft 7 extends outwardly through the rear end 6 and has a drive, not shown, connected thereto. The support frame 2 which performs the function of supporting the drum 1 for rotation cooperates with the outer periphery 8 to define therebetween a hot blast inlet duct 10 to supply a hot blast therethrough into the interior of the drum 1. When the drum 1 is angularly rotated about the air inlet tube 3 and air outlet tube 4 to withdraw the tablets from the drum 1, the support frame 2 can be made to face downwardly. More specifically, the air inlet tube 3 and air outlet tube 4 have the dual function of supporting the drum 1 or support frame 2 for angular rotation backwardly and forwardly and of supplying a hot blast to the duct 10, and are connected to the support frame 2 (drum 1) on a horizontal line extending through the center thereof. The numeral 9 designates spray nozzles for spraying the solution to the tablets.
In the apparatus for applying a coating to tablets of the prior art constructed as described hereinabove, the air inlet tube 3 and air outlet tube 4 are connected to the support frame 2 on a horizontal line extending through the center of the drum 1, and a tablet coating operation is carried out while the drum 1 is maintained in a vertical position. Some disadvantages are associated with the tablet coating apparatus of the prior art.
In the aspect of construction, the apparatus including the support frame and the drive becomes unbalanced and the rear side becomes overweighted because the drive including a motor and a speed change gearing is mounted on the rear end of the drum 1. This makes it necessary to cause the air inlet tube 3 and air outlet tube 4 to produce a force urging them to resist the tendency of the drum 1 to become unbalanced, to maintain the drum 1 in a vertical position at all times. Thus the construction that would enable the balancing of the drum 1 to be achieved as aforesaid would have to bear a heavy burden, with the result that the apparatus itself would have a short service life. In addition, when the support frame 2 is caused to face downwardly in withdrawing the tablets from the drum 1, a force of high magnitude is required. In the aspect of operation, the tablets T placed in the drum 1 strongly tend to move along an orbit which is parallel to the direction of rotation of the drum 1. Thus there is a considerable difference in speed between the mixing of the tablets T moving along the same orbit and mixing of the tablets T moving along different orbits. For example, the tablets Ta located near the center of the drum 1 mix one another at the early stages of rotation of the drum 1, but the tablets Tc located at the opposite ends of the drum 1 have difficulty in being mixed with one another. Thus irregularities and variations in drying and coating of the tablets T tend to occur. To avoid this phenomenon, it becomes necessary to prolong the time of operation so that spraying of the tablets with a solution can be performed slowly. In this system of operation, the number of spray nozzles for spraying the solution should be minimized because the layers of tablets in the drum that move along the orbits therein have a narrow width. This results in a reduction in operation efficiency.